Understanding Rules File

[Ryan McClue <re.mcclue@protonmail.com>]

At a high level I understand that the debian/rules file instructs debhelper as to how to build your package. However, I'm confused regarding these two points:

1. Why is debhelper used? Why not just explicitly state in the debian/rules file how to build your package with either direct shell, e.g. gcc file.c -o file, or with a buildtool, e.g. cmake --build . etc. Why the level of indirection?

2. How does debhelper work? Is there a way I can see what specific commands it is executing, e.g. what does the default %: dh $@ actually equate to?

Thanks